You have to create a second element
XmlElement xDCTObalance_2 = XDoc.CreateElement("OPENING-BALANCE");
xDCTObalance_2.SetAttribute("Detail", "AED_TTA");
xDCTObalance_2.InnerText = "0";
XDoc.DocumentElement.AppendChild(xDCTObalance_2);
You override the former information ;)